There are now a number of applications in which it is required to label antibodies (or other organic substances, for example macromolecules such as proteins, and haptens) with metal ions, either radioactive metal ions for use in radioimmunoassay and other nuclear medicine studies or lanthanide metal ions for fluorometric immunoassay and other studies involving fluorescence. For these purposes the organic substances are conventionally labelled with metal ions through the agency of chelates. Hitherto the chelates have been modified with bridging reagents which convert them into bifunctional reagents so that they retain their chelation function whilst being readily attachable by covalent bonding to the molecule to be labelled.
One class of chelating reagents which has been used for this purpose is the class of .beta.-diketones, such as trifluoroacetylacetone and benzoyl and .alpha.- and .beta.-naphthoyl trifluoroacetone, and chelates of lanthanide metal ions with such reagents have been coupled to antibodies using EDTA-analogues (see European patent application No. 0.064,484). It has also been proposed in GB Patent Specification No. 1,560,402 to modify a .beta.-diketone ligand such as thenoyl-trifluoroacetylacetone by the attachment of an aminomethyl substituent and to use one molecule of the modified ligand and two molecules of unmodified ligand to form a lanthanide metal ion complex which readily couples to an antibody after conversion of the amino group to an isothiocyanate group.
However, bifunctional-chelating agents are difficult to synthesize and many of the reactions by which covalent bonding of the ligand-metal ion complex to the molecule to be labelled is achieved have only a low yield of the desired labelled product and may also confer undesirable properties on the labelled molecule.